Coast & Country Construction and Paul Humphries Architects have been fined a total of £170,000.
Exeter Magistrates' Court was told that in 2016 a concern was raised about dangerous work practices at Manor Lodge Residential Home in Exmouth, Devon, where a large timber frame extension was being built.
Inspectors from the Health and Safety Executive (HSE) visited the site on 1 March that year and found workers were at risk of falls from height, slips and trips and wood dust exposure.
In addition, the construction of the timber extension adjoining the existing building created a risk of fire spread that could have injured or killed the care home's vulnerable residents.
A subsequent investigation by the HSE found the work had not been properly planned, supervised or carried out safely.
It said it was Coast & Country Construction's duty as the principal contractor to control how the work was carried out and to ensure that it would be completed safely, while principal designer Paul Humphries Architects had failed to consider the risk of fire spreading.
Coast & Country Construction did not attend court but was found guilty in its absence to breaching ss 2(1) and 3(1) of the Health and Safety and Work Act. It was fined £150,000.
Paul Humphries Architects pleaded guilty to breaching regs 11(1) and 11(3) of the Construction (Design and Management) Regulations 2015, which cover the duties of a principal designer at the pre-construction phase. It was fined £20,000.
Both companies have been ordered to pay costs of £6,039 each.
After the hearing HSE inspector Nicole Buchanan said: "Those in control of work have a responsibility to devise safe methods of working and to provide the necessary information, instruction and training to their workers in the safe system of working."